Transversely-excited film bulk acoustic resonator and filter with a uniform-thickness dielectric overlayer

ABSTRACT

Acoustic filters, resonators and methods are disclosed. An acoustic filter device includes a substrate having a surface and a single-crystal piezoelectric plate having front and back surfaces and a thickness ts, the back surface attached to the surface of the substrate except for portions of the piezoelectric plate forming a plurality of diaphragms that span respective cavities in the substrate. A conductor pattern is formed on the front surface of the piezoelectric plate, the conductor pattern comprising a plurality of interdigital transducers (IDTs) of a plurality of acoustic resonators, interleaved fingers of each IDT of the plurality of IDTs disposed on a respective diaphragm of the plurality of diaphragms. Zero or more dielectric layers are deposited over all of the IDTs and the diaphragms, wherein a total thickness of the zero or more dielectric layers is the same for all of the plurality of acoustic resonators.

RELATED APPLICATION INFORMATION

This patent claims priority from provisional patent application62/904,233, filed Sep. 23, 2019, entitled XBAR FILTERS USING RESONATORSWITH SMALL PITCHES TO ACHIEVE FREQUENCY SEPARATION. This patent is acontinuation-in-part of application Ser. No. 16/920,173, titledTRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR, filed Jul. 2, 2020,which is a continuation of application Ser. No. 16/438,121 titledTRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR, filed Jun. 11, 2019,now U.S. Pat. No. 10,756,697, which is a continuation-in-part ofapplication Ser. No. 16/230,443, entitled TRANSVERSELY-EXCITED FILM BULKACOUSTIC RESONATOR, filed Dec. 21, 2018, now U.S. Pat. No. 10,491,192,which claims priority from the following provisional patentapplications: application 62/685,825, filed Jun. 15, 2018, entitledSHEAR-MODE FBAR (XBAR); application 62/701,363, filed Jul. 20, 2018,entitled SHEAR-MODE FBAR (XBAR); application 62/741,702, filed Oct. 5,2018, entitled 5 GHZ LATERALLY-EXCITED BULK WAVE RESONATOR (XBAR);application 62/748,883, filed Oct. 22, 2018, entitled SHEAR-MODE FILMBULK ACOUSTIC RESONATOR; and application 62/753,815, filed Oct. 31,2018, entitled LITHIUM TANTALATE SHEAR-MODE FILM BULK ACOUSTICRESONATOR. All of these applications are incorporated herein byreference.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by anyone of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

BACKGROUND Field

This disclosure relates to radio frequency filters using acoustic waveresonators, and specifically to filters for use in communicationsequipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to passsome frequencies and to stop other frequencies, where “pass” meanstransmit with relatively low signal loss and “stop” means block orsubstantially attenuate. The range of frequencies passed by a filter isreferred to as the “pass-band” of the filter. The range of frequenciesstopped by such a filter is referred to as the “stop-band” of thefilter. A typical RF filter has at least one pass-band and at least onestop-band. Specific requirements on a passband or stop-band depend onthe specific application. For example, a “pass-band” may be defined as afrequency range where the insertion loss of a filter is better than adefined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be definedas a frequency range where the rejection of a filter is greater than adefined value such as 20 dB, 30 dB, 40 dB, or greater depending onapplication. RF filters are used in communications systems whereinformation is transmitted over wireless links. For example, RF filtersmay be found in the RF front-ends of cellular base stations, mobiletelephone and computing devices, satellite transceivers and groundstations, IoT (Internet of Things) devices, laptop computers andtablets, fixed point radio links, and other communications systems. RFfilters are also used in radar and electronic and information warfaresystems.

RF filters typically require many design trade-offs to achieve, for eachspecific application, the best compromise between performance parameterssuch as insertion loss, rejection, isolation, power handling, linearity,size and cost. Specific design and manufacturing methods andenhancements can benefit simultaneously one or several of theserequirements.

Performance enhancements to the RF filters in a wireless system can havebroad impact to system performance. Improvements in RF filters can beleveraged to provide system performance improvements such as larger cellsize, longer battery life, higher data rates, greater network capacity,lower cost, enhanced security, higher reliability, etc. Theseimprovements can be realized at many levels of the wireless system bothseparately and in combination, for example at the RF module, RFtransceiver, mobile or fixed sub-system, or network levels.

The desire for wider communication channel bandwidths will inevitablylead to the use of higher frequency communications bands. The currentLTE™ (Long Term Evolution) specification defines frequency bands from3.3 GHz to 5.9 GHz. These bands are not presently used. Future proposalsfor wireless communications include millimeter wave communication bandswith frequencies up to 28 GHz.

High performance RF filters for present communication systems commonlyincorporate acoustic wave resonators including surface acoustic wave(SAW) resonators, bulk acoustic wave (BAW) resonators, film bulkacoustic wave resonators (FBAR), and other types of acoustic resonators.However, these existing technologies are not well-suited for use at thehigher frequencies proposed for future communications networks.

DESCRIPTION OF THE DRAWINGS

FIG. 1 has a schematic plan view and two schematic cross-sectional viewsof a transversely-excited film bulk acoustic resonator (XBAR).

FIG. 2 is an expanded schematic cross-sectional view of a portion of theXBAR of FIG. 1.

FIG. 3 is an alternative schematic cross-sectional view of the XBAR ofFIG. 1.

FIG. 4 is a graphic illustrating a shear primary acoustic mode in anXBAR.

FIG. 5 is a schematic block diagram of a filter using XBARs.

FIG. 6 is a schematic cross-sectional view of two XBARs illustrating afrequency-setting dielectric layer.

FIG. 7 is a graph identifying preferred combinations of aluminum IDTthickness and IDT pitch for XBARs with thin aluminum conductors.

FIG. 8 is graph of resonance frequency as a function of IDT thicknessand IDT pitch.

FIG. 9 is graph of coupling factor Γ as a function of IDT thickness andIDT pitch.

FIG. 10 is a graph of input-output transfer function (S21) versusfrequency for a filter using XBARs with thin aluminum IDT conductors andwithout a frequency-setting dielectric layer.

FIG. 11 is a flow chart of a process for fabricating an acousticresonator filter without frequency-setting dielectric layers.

Throughout this description, elements appearing in figures are assignedthree-digit or four-digit reference designators, where the two leastsignificant digits are specific to the element and the one or two mostsignificant digit is the figure number where the element is firstintroduced. An element that is not described in conjunction with afigure may be presumed to have the same characteristics and function asa previously-described element having the same reference designator.

DETAILED DESCRIPTION

Description of Apparatus

FIG. 1 shows a simplified schematic top view and orthogonalcross-sectional views of a transversely-excited film bulk acousticresonator (XBAR) 100. XBAR resonators such as the resonator 100 may beused in a variety of RF filters including band-reject filters, band-passfilters, duplexers, and multiplexers. XBARs are well suited for use infilters for communications bands with frequencies above 3 GHz.

The XBAR 100 is made up of a thin film conductor pattern formed on asurface of a piezoelectric plate 110 having a front surface 112 and aback surface 114. The front and back surfaces are essentially parallel.“Essentially parallel” means parallel to the extent possible withinnormal manufacturing tolerances. The piezoelectric plate is a thinsingle-crystal layer of a piezoelectric material such as lithiumniobate, lithium tantalate, lanthanum gallium silicate, gallium nitride,or aluminum nitride. The piezoelectric plate is cut such that theorientation of the X, Y, and Z crystalline axes with respect to thefront and back surfaces is known and consistent. In the examplespresented in this patent, the piezoelectric plates are Z-cut, which isto say the Z axis is normal to the front surface 112 and back surface114. However, XBARs may be fabricated on piezoelectric plates with othercrystallographic orientations including rotated Z-cut and rotatedYX-cut.

The back surface 114 of the piezoelectric plate 110 is attached to asurface 122 of the substrate 120 except for a portion of thepiezoelectric plate 110 that forms a diaphragm 115 spanning a cavity 140formed in the substrate 120. The portion of the piezoelectric plate thatspans the cavity is referred to herein as the “diaphragm” due to itsphysical resemblance to the diaphragm of a microphone. As shown in FIG.1, the diaphragm 115 is contiguous with the rest of the piezoelectricplate 110 around all of a perimeter 145 of the cavity 140. In thiscontext, “contiguous” means “continuously connected without anyintervening item”.

The substrate 120 provides mechanical support to the piezoelectric plate110. The substrate 120 may be, for example, silicon, sapphire, quartz,or some other material or combination of materials. The back surface 114of the piezoelectric plate 110 may be attached to the substrate 120using a wafer bonding process. Alternatively, the piezoelectric plate110 may be grown on the substrate 120 or otherwise attached to thesubstrate. The piezoelectric plate 110 may be attached directly to thesubstrate or may be attached to the substrate 120 via one or moreintermediate material layers.

The cavity 140 is an empty space within a solid body of the resonator100. The cavity 140 may be a hole completely through the substrate 120(as shown in Section A-A and Section B-B) or a recess in the substrate120 (as shown subsequently in FIG. 3A and FG. 3B). The cavity 140 may beformed, for example, by selective etching of the substrate 120 before orafter the piezoelectric plate 110 and the substrate 120 are attached.

The conductor pattern of the XBAR 100 includes an interdigitaltransducer (IDT) 130. An IDT is an electrode structure for convertingbetween electrical and acoustic energy in piezoelectric devices. The IDT130 includes a first plurality of parallel elongated conductors,commonly called “fingers”, such as finger 136, extending from a firstbusbar 132. The IDT 130 includes a second plurality of fingers extendingfrom a second busbar 134. The first and second pluralities of parallelfingers are interleaved. The interleaved fingers overlap for a distanceAP, commonly referred to as the “aperture” of the IDT. Thecenter-to-center distance L between the outermost fingers of the IDT 130is the “length” of the IDT.

The term “busbar” refers to the conductors that interconnect the firstand second sets of fingers in an IDT. As shown in FIG. 1, each busbar132, 134 is an elongated rectangular conductor with a long axisorthogonal to the interleaved fingers and having a length approximatelyequal to the length L of the IDT. The busbars of an IDT need not berectangular or orthogonal to the interleaved fingers and may havelengths longer than the length of the IDT.

The first and second busbars 132, 134 serve as the terminals of the XBAR100. A radio frequency or microwave signal applied between the twobusbars 132, 134 of the IDT 130 excites a primary acoustic mode withinthe piezoelectric plate 110. As will be discussed in further detail, theprimary acoustic mode is a bulk shear mode where acoustic energypropagates along a direction substantially orthogonal to the surface ofthe piezoelectric plate 110, which is also normal, or transverse, to thedirection of the electric field created by the IDT fingers. Thus, theXBAR is considered a transversely-excited film bulk wave resonator.

The IDT 130 is positioned on the piezoelectric plate 110 such that atleast the fingers of the IDT 130 are disposed on the diaphragm 115 ofthe piezoelectric plate that spans, or is suspended over, the cavity140. As shown in FIG. 1, the cavity 140 has a rectangular shape with anextent greater than the aperture AP and length L of the IDT 130. Acavity of an XBAR may have a different shape, such as a regular orirregular polygon. The cavity of an XBAR may more or fewer than foursides, which may be straight or curved.

For ease of presentation in FIG. 1, the geometric pitch and width of theIDT fingers is greatly exaggerated with respect to the length (dimensionL) and aperture (dimension AP) of the XBAR. An XBAR for a 5G device willhave more than ten parallel fingers in the IDT 110. An XBAR may havehundreds, possibly thousands, of parallel fingers in the IDT 110.Similarly, the thickness of the fingers in the cross-sectional views isgreatly exaggerated in the drawings.

FIG. 2 shows a detailed schematic cross-sectional view of the XBAR 100.The piezoelectric plate 110 is a single-crystal layer of piezoelectricalmaterial having a thickness ts. ts may be, for example, 100 nm to 1500nm. When used in filters for LTE™ bands from 3.4 GHZ to 6 GHz (e.g.bands 42, 43, 46), the thickness ts may be, for example, 200 nm to 1000nm.

A front-side dielectric layer 214 may be formed on the front side of thepiezoelectric plate 110. The “front side” of the XBAR is the surfacefacing away from the substrate. The front-side dielectric layer 214 hasa thickness tfd. The front-side dielectric layer 214 is formed betweenthe IDT fingers 238. Although not shown in FIG. 2, the front sidedielectric layer 214 may also be deposited over the IDT fingers 238. Aback-side dielectric layer 216 may be formed on the back side of thepiezoelectric plate 110. The back-side dielectric layer 216 has athickness tbd. The front-side and back-side dielectric layers 214, 216may be a non-piezoelectric dielectric material, such as silicon dioxideor silicon nitride. tfd and tbd may be, for example, 0 to 500 nm. tfdand tbd are typically less than the thickness ts of the piezoelectricplate. tfd and tbd are not necessarily equal, and the front-side andback-side dielectric layers 214, 216 are not necessarily the samematerial. Either or both of the front-side and back-side dielectriclayers 214, 216 may be formed of multiple layers of two or morematerials.

The IDT fingers 238 may be one or more layers of aluminum, asubstantially aluminum alloys, copper, a substantially copper alloys,beryllium, gold, molybdenum, or some other conductive material. Thin(relative to the total thickness of the conductors) layers of othermetals, such as chromium or titanium, may be formed under and/or overthe fingers to improve adhesion between the fingers and thepiezoelectric plate 110 and/or to passivate or encapsulate the fingers.The busbars (132, 134 in FIG. 1) of the IDT may be made of the same ordifferent materials as the fingers. As shown in FIG. 2, the IDT fingers238 have rectangular cross-sections. The IDT fingers may have some othercross-sectional shape, such as trapezoidal.

Dimension p is the center-to-center spacing or “pitch” of the IDTfingers, which may be referred to as the pitch of the IDT and/or thepitch of the XBAR. Dimension w is the width or “mark” of the IDTfingers. The IDT of an XBAR differs substantially from the IDTs used insurface acoustic wave (SAW) resonators. In a SAW resonator, the pitch ofthe IDT is one-half of the acoustic wavelength at the resonancefrequency. Additionally, the mark-to-pitch ratio of a SAW resonator IDTis typically close to 0.5 (i.e., the mark or finger width is aboutone-fourth of the acoustic wavelength at resonance). In an XBAR, thepitch p of the IDT is typically 2 to 20 times the width w of thefingers. In addition, the pitch p of the IDT is typically 2 to 20 timesthe thickness ts of the piezoelectric slab 212. The width of the IDTfingers in an XBAR is not constrained to one-fourth of the acousticwavelength at resonance. For example, the width of XBAR IDT fingers maybe 500 nm or greater, such that the IDT can be fabricated using opticallithography. The thickness tm of the IDT fingers may be from 100 nm toabout equal to the width w. The thickness of the busbars (132, 134 inFIG. 1) of the IDT may be the same as, or greater than, the thickness tmof the IDT fingers.

FIG. 3 is an alternative cross-sectional view along the section planeA-A defined in FIG. 1. In FIG. 3, a piezoelectric plate 310 is attachedto a substrate 320. A portion of the piezoelectric plate 310 forms adiaphragm 315 spanning a cavity 340 in the substrate. The cavity 340does not fully penetrate the substrate 320. Fingers of an IDT aredisposed on the diaphragm 315. The cavity 340 may be formed, forexample, by etching the substrate 320 before attaching the piezoelectricplate 310. Alternatively, the cavity 340 may be formed by etching thesubstrate 320 with a selective etchant that reaches the substratethrough one or more openings (not shown) provided in the piezoelectricplate 310. In this case, the diaphragm 315 may be contiguous with therest of the piezoelectric plate 310 around a large portion of aperimeter 345 of the cavity 340. For example, the diaphragm 315 may becontiguous with the rest of the piezoelectric plate 310 around at least50% of the perimeter 345 of the cavity 340.

FIG. 4 is a graphical illustration of the primary acoustic mode ofinterest in an XBAR. FIG. 4 shows a small portion of an XBAR 400including a piezoelectric plate 410 and three interleaved IDT fingers430. A radio frequency (RF) voltage is applied to the interleavedfingers 430. This voltage creates a time-varying electric field betweenthe fingers. The direction of the electric field is primarily lateral,or parallel to the surface of the piezoelectric plate 410, as indicatedby the arrows labeled “electric field”. Since the dielectric constant ofthe piezoelectric plate is significantly higher than the surroundingair, the electric field is highly concentrated in the plate relative tothe air. The lateral electric field introduces shear deformation, andthus strongly excites a shear-mode acoustic mode, in the piezoelectricplate 410. Shear deformation is deformation in which parallel planes ina material remain parallel and maintain a constant distance whiletranslating relative to each other. A “shear acoustic mode” is anacoustic vibration mode in a medium that results in shear deformation ofthe medium. The shear deformations in the XBAR 400 are represented bythe curves 460, with the adjacent small arrows providing a schematicindication of the direction and magnitude of atomic motion. The degreeof atomic motion, as well as the thickness of the piezoelectric plate410, have been greatly exaggerated for ease of visualization. While theatomic motions are predominantly lateral (i.e. horizontal as shown inFIG. 4), the direction of acoustic energy flow of the excited primaryshear acoustic mode is substantially orthogonal to the surface of thepiezoelectric plate, as indicated by the arrow 465.

An acoustic resonator based on shear acoustic wave resonances canachieve better performance than current state-of-the artfilm-bulk-acoustic-resonators (FBAR) and solidly-mounted-resonatorbulk-acoustic-wave (SMR BAW) devices where the electric field is appliedin the thickness direction. In such devices, the acoustic mode iscompressive with atomic motions and the direction of acoustic energyflow in the thickness direction. In addition, the piezoelectric couplingfor shear wave XBAR resonances can be high (>20%) compared to otheracoustic resonators. High piezoelectric coupling enables the design andimplementation of microwave and millimeter-wave filters with appreciablebandwidth.

FIG. 5 is a schematic circuit diagram and layout for a high frequencyband-pass filter 500 using XBARs. The filter 500 has a conventionalladder filter architecture including three series resonators 510A, 510B,510C and two shunt resonators 520A, 520B. The three series resonators510A, 510B, and 510C are connected in series between a first port and asecond port (hence the term “series resonator”). In FIG. 5, the firstand second ports are labeled “In” and “Out”, respectively. However, thefilter 500 is bidirectional and either port may serve as the input oroutput of the filter. The two shunt resonators 520A, 520B are connectedfrom nodes between the series resonators to ground. A filter may containadditional reactive components, such as inductors, not shown in FIG. 5.All the shunt resonators and series resonators are XBARs. The inclusionof three series and two shunt resonators is exemplary. A filter may havemore or fewer than five total resonators, more or fewer than threeseries resonators, and more or fewer than two shunt resonators.Typically, all of the series resonators are connected in series betweenan input and an output of the filter. All of the shunt resonators aretypically connected between ground and the input, the output, or a nodebetween two series resonators.

In the exemplary filter 500, the three series resonators 510A, B, C andthe two shunt resonators 520A, B of the filter 500 are formed on asingle plate 530 of piezoelectric material bonded to a silicon substrate(not visible). Each resonator includes a respective IDT (not shown),with at least the fingers of the IDT disposed over a cavity in thesubstrate. In this and similar contexts, the term “respective” means“relating things each to each”, which is to say with a one-to-onecorrespondence. In FIG. 5, the cavities are illustrated schematically asthe dashed rectangles (such as the rectangle 535). In this example, eachIDT is disposed over a respective cavity. In other filters, the IDTs oftwo or more resonators may be disposed over a single cavity.

Each of the resonators 510A, 510B, 510C, 520A, 520B in the filter 500has resonance where the admittance of the resonator is very high and ananti-resonance where the admittance of the resonator is very low. Theresonance and anti-resonance occur at a resonance frequency and ananti-resonance frequency, respectively, which may be the same ordifferent for the various resonators in the filter 500. Inover-simplified terms, each resonator can be considered a short-circuitat its resonance frequency and an open circuit at its anti-resonancefrequency. The input-output transfer function will be near zero at theresonance frequencies of the shunt resonators and at the anti-resonancefrequencies of the series resonators. In a typical filter, the resonancefrequencies of the shunt resonators are positioned below the lower edgeof the filter's passband and the anti-resonance frequencies of theseries resonators are position above the upper edge of the passband.

FIG. 6 is a schematic cross-sectional view through a shunt resonator anda series resonator of a filter 600 that uses a dielectric frequencysetting layer to separate the resonance frequencies of shunt and seriesresonators. A piezoelectric plate 610 is attached to a substrate 620.Portions of the piezoelectric plate 610 form diaphragms spanningcavities 640 in the substrate 620. Interleaved IDT fingers, such asfinger 630, are formed on the diaphragms. A first dielectric layer 650,having a thickness t1, is formed over the IDT of the shunt resonator.The first dielectric layer 650 is considered a “frequency settinglayer”, which is a layer of dielectric material applied to a firstsubset of the resonators in a filter to offset the resonance frequenciesof the first subset of resonators with respect to the resonancefrequencies of resonators that do not receive the dielectric frequencysetting layer. The dielectric frequency setting layer is commonly SiO₂but may be silicon nitride, aluminum oxide, or some other dielectricmaterial. The dielectric frequency setting layer may be a laminate orcomposite of two or more dielectric materials.

A second dielectric layer 655, having a thickness t2, may be depositedover both the shunt and series resonator. The second dielectric layer655 serves to seal and passivate the surface of the filter 600. Thesecond dielectric layer may be the same material as the first dielectriclayer or a different material. The second dielectric layer may be alaminate or composite of two or more different dielectric materials.Further, as will be described subsequently, the thickness of the seconddielectric layer may be locally adjusted to fine-tune the frequency ofthe filter 600. Thus, the second dielectric layer can be referred to asthe “passivation and tuning layer”.

The resonance frequency of an XBAR is roughly proportional to theinverse of the total thickness of the diaphragm including thepiezoelectric plate 610 and the dielectric layers 650, 655. Thediaphragm of the shunt resonator is thicker than the diaphragm of theseries resonator by the thickness t1 of the dielectric frequency settinglayer 650. Thus, the shunt resonator will have a lower resonancefrequency than the series resonator. The difference in resonancefrequency between series and shunt resonators is determined by thethickness t1.

U.S. Pat. No. 10,637,438 describes XBAR resonators for use in high powerapplications. For such applications, the conductors of the IDT are verythick to remove heat from the resonator diaphragm. The conductors maybe, for example, aluminum with a thickness between 0.85 times thediaphragm thickness and 2.5 times the diaphragm thickness. With suchthick conductors, the preferred range for the pitch of the IDT fingersis from 6 times the diaphragm thickness to 12.5 times the diaphragmthickness. Varying the pitch of the IDT fingers over this range allowstuning the resonance frequency of an XBAR over a limited range. Theresonance frequency of an XBAR with a pitch equal to 6 times thediaphragm thickness is about 4.5% higher than the resonance frequency ofan XBAR with a pitch equal to 12.5 times the diaphragm thickness.

U.S. Pat. No. 10,637,438 also describes the use of a figure of merit(FOM) to define a design space (i.e. combinations of IDT conductorthickness, pitch and width) that provides XBARs with acceptableperformance for use in filters. The FOM is calculated by integrating thenegative impact of spurious modes across a defined frequency range. Foreach combination of IDT conductor thickness and pitch, the FOM iscalculated for a range of IDT finger widths. The minimum FOM value overthe range of IDT finger widths is considered the minimized FOM for thatconductor thickness/pitch combination. The definition of the FOM and thefrequency range depend on the requirements of a particular filter. Thefrequency range typically includes the passband of the filter and mayinclude one or more stop bands. Spurious modes occurring between theresonance and anti-resonance frequencies of each hypothetical resonatormay be accorded a heavier weight in the FOM than spurious modes atfrequencies below resonance or above anti-resonance. Hypotheticalresonators having a minimized FOM below a threshold value wereconsidered potentially “useable”, which is to say probably havingsufficiently low spurious modes for use in a filter. Hypotheticalresonators having a minimized cost function above the threshold valuewere considered not useable.

FOM calculations showed that useable resonators were possible over awide range of IDT pitch for relatively thin IDT conductors in athickness range of 50 nm to 150 nm. Such resonators may not be suitablefor high power applications but may be useful for filters inreceive-only communications channels and other applications.

FIG. 7 is a chart 700 showing combinations of IDT pitch and IDT fingerthickness that may provide useable resonators. Both IDT pitch and IDTfinger thickness are normalized to the thickness of the diaphragm. Thischart is based on two-dimensional simulations of XBARs with lithiumniobate diaphragms, aluminum conductors, no dielectric tuning layer, anda typical passivation layer. XBARs with IDT pitch and thickness withinunshaded regions 710, 720, 730 are likely to have sufficiently lowspurious effects for use in filters. XBARs with IDT pitch and thicknesswithin the intervening shaded regions have unacceptably high spuriousmodes for use in the target filter. With IDT conductor thickness lessthan 0.33 times the diaphragm thickness, usable resonators exist for IDTpitch values within ranges of 2.8 to 3.6 times the diaphragm thickness,5.0 to 6.8 times the diaphragm thickness, and 8.7 to 10.2 times thediaphragm thickness. These ranges are progressively wider for thinnerIDT conductors. For IDT conductor thickness less than or equal to 0.225times the diaphragm thickness, usable resonators exist for IDT pitchvalues within ranges of 2.8 to 3.9 times the diaphragm thickness, 5.0 to6.9 times the diaphragm thickness, and 8.3 to 10.3 times the diaphragmthickness. For IDT conductor thickness less than or equal to 0.14 timesthe diaphragm thickness, usable resonators exist for IDT pitch valueswithin ranges of 2.8 to 4.0 times the diaphragm thickness, 5.0 to 7.2times the diaphragm thickness, and 8.2 to 10.6 times the diaphragmthickness.

FIG. 8 is a graph 800 of XBAR resonance frequency as a function of IDTpitch and IDT finger thickness. The labeled contour lines indicate theresonance frequency of XBARs with 440 nm thick lithium niobatediaphragm, aluminum conductors, no dielectric tuning layer, and atypical passivation layer. The erratic shapes of some of the contourlines are due to spurious modes passing near the resonance frequency.This graph is based on two-dimensional simulation using a finite elementmethod.

XBARs with IDT pitch/diaphragm thickness within unshaded regions 810,820, 830 (which correspond to the unshaded regions 710, 720, 730 of FIG.7) are likely to have sufficiently low spurious effects for use infilters. XBARs with IDT pitch and thickness within the interveningshaded regions have unacceptably high spurious modes for use in thetarget filter. FIG. 8 illustrates that, for XBAR resonators with thinIDT electrodes, it is possible to tune the resonance frequency of over arange of more than 15% using the pitch of the IDT fingers. For example,the resonance frequency of an XBAR with a 440 nm diaphragm thickness canbe set between about 4100 MHz to more than 4800 MHz by varying the IDTpitch.

The separation between the resonance and anti-resonance frequencies ofan acoustic resonator is determined by the electromechanical couplingfactor Γ (gamma), which is dependent on the pitch of the IDT. Gamma is ametric defined by the equation:

$\Gamma = \frac{1}{\left( {{Fa}\text{/}Fr} \right)^{2} - 1}$where Fa is the antiresonance frequency and Fr is the resonancefrequency. Large values for gamma correspond to smaller separationbetween the resonance and anti-resonance frequencies. Low values ofgamma indicate larger separation between the resonance andanti-resonance frequencies, which is good for wideband ladder filters.

FIG. 9 is a graph 900 of electromechanical coupling factor Γ as afunction of IDT pitch and IDT finger thickness. The labeled contourlines indicate the Γ of XBARs with 440 nm thick lithium niobatediaphragm, aluminum conductors, no dielectric tuning layer, and atypical passivation layer. The erratic shapes of some of the contourlines are due to spurious modes passing near the resonance oranti-resonance frequencies. This graph is based on two-dimensionalsimulation using a finite element method.

XBARs with IDT pitch/diaphragm thickness within unshaded regions 910,920, 930 (which correspond to the unshaded regions 710, 720, 730 of FIG.7) are likely to have sufficiently low spurious effects for use infilters. XBARs with IDT pitch and thickness within the interveningshaded regions have unacceptably high spurious modes for use in thetarget filter. XBARs with IDT pitch/diaphragm thickness within unshadedregion 910 have Γ of 3.6 or less. XBARs with IDT pitch/diaphragmthickness within unshaded region 920 have Γ between 3.8 and 4.6. XBARswith IDT pitch/diaphragm thickness within unshaded region 930 have Γequal to or greater than 5.0.

XBARs with thin aluminum conductors can be tuned using IDT pitch over atuning range of more than 15%. Such XBARs will have Γ less than 3.6 atthe lower frequency end of this tuning range and Γ about 6.5 at theupper end of the tuning range. These resonator characteristics aresufficient for to implement a variety of bandpass filter applications,including filters with 15% or larger bandwidth, without the inclusion ofa dielectric frequency setting layer. Such filters may have none, one,or more dielectric layers deposited over all of the IDTs and diaphragms.Prior to tuning, the total deposited thickness of the dielectrics layersis the same for all resonators. The dielectric layer(s) may be orinclude a passivation and tuning layer as previously described.

FIG. 10 is a graph 1000 of the measured input-output transfer function(S21) of an initial experimental filter using five XBAR devices asdescribed previously. Specifically, the line 1010 is a plot of themagnitude of S21 as a function of frequency. This experimental filter isnot intended to meet the requirements of any particular communicationsband. The center of the pass band is 4665 MHz and the −3 dB bandwidth is440 MHz. Further optimization of the filter design may result inimprovements such as increased bandwidth, reduced insertion loss in thepassband, increased attenuation outside of the pass band, and reducedspurious effects.

Description of Methods

FIG. 11 is a simplified flow chart showing a process 1100 for making anXBAR or a filter incorporating XBARs. The process 1100 starts at 1105with a substrate and a plate of piezoelectric material and ends at 1195with a completed XBAR or filter. The flow chart of FIG. 11 includes onlymajor process steps. Various conventional process steps (e.g. surfacepreparation, cleaning, inspection, baking, annealing, monitoring,testing, etc.) may be performed before, between, after, and during thesteps shown in FIG. 11.

The flow chart of FIG. 11 captures three variations of the process 1100for making an XBAR which differ in when and how cavities are formed inthe substrate. The cavities may be formed at steps 1110A, 1110B, or1110C. Only one of these steps is performed in each of the threevariations of the process 1100.

The piezoelectric plate may be, for example, Z-cut lithium niobate orlithium tantalate as used in the previously presented examples. Thepiezoelectric plate may be some other material and/or some other cut.The substrate may preferably be silicon. The substrate may be some othermaterial that allows formation of deep cavities by etching or otherprocessing.

In one variation of the process 1100, one or more cavities are formed inthe substrate at 1110A before the piezoelectric plate is bonded to thesubstrate at 1120. A separate cavity may be formed for each resonator ina filter device. The one or more cavities may be formed usingconventional photolithographic and etching techniques. Typically, thecavities formed at 1110A will not penetrate through the substrate, andthe resulting resonator devices will have a cross-section as shown inFIG. 3.

At 1120, the piezoelectric plate is bonded to the substrate. Thepiezoelectric plate and the substrate may be bonded by a wafer bondingprocess. Typically, the mating surfaces of the substrate and thepiezoelectric plate are highly polished. One or more layers ofintermediate materials, such as an oxide or metal, may be formed ordeposited on the mating surface of one or both of the piezoelectricplate and the substrate. One or both mating surfaces may be activatedusing, for example, a plasma process. The mating surfaces may then bepressed together with considerable force to establish molecular bondsbetween the piezoelectric plate and the substrate or intermediatematerial layers.

A conductor pattern, including IDTs of each XBAR, is formed at 1130 bydepositing and patterning one or more conductor layers on the front sideof the piezoelectric plate. The conductor layer may be, for example,aluminum, an aluminum alloy, copper, a copper alloy, or some otherconductive metal. Optionally, one or more layers of other materials maybe disposed below (i.e. between the conductor layer and thepiezoelectric plate) and/or on top of the conductor layer. For example,a thin film of titanium, chrome, or other metal may be used to improvethe adhesion between the conductor layer and the piezoelectric plate. Aconduction enhancement layer of gold, aluminum, copper or other higherconductivity metal may be formed over portions of the conductor pattern(for example the IDT bus bars and interconnections between the IDTs).

The conductor pattern may be formed at 1130 by depositing the conductorlayer and, optionally, one or more other metal layers in sequence overthe surface of the piezoelectric plate. The excess metal may then beremoved by etching through patterned photoresist. The conductor layercan be etched, for example, by plasma etching, reactive ion etching, wetchemical etching, and other etching techniques.

Alternatively, the conductor pattern may be formed at 1130 using alift-off process. Photoresist may be deposited over the piezoelectricplate. and patterned to define the conductor pattern. The conductorlayer and, optionally, one or more other layers may be deposited insequence over the surface of the piezoelectric plate. The photoresistmay then be removed, which removes the excess material, leaving theconductor pattern.

In a second variation of the process 1100, one or more cavities areformed in the back side of the substrate at 1110B. A separate cavity maybe formed for each resonator in a filter device. The one or morecavities may be formed using an anisotropic or orientation-dependent dryor wet etch to open holes through the back side of the substrate to thepiezoelectric plate. In this case, the resulting resonator devices willhave a cross-section as shown in FIG. 1.

In a third variation of the process 1100, one or more cavities in theform of recesses in the substrate may be formed at 1110C by etching thesubstrate using an etchant introduced through openings in thepiezoelectric plate. A separate cavity may be formed for each resonatorin a filter device. The one or more cavities formed at 1110C will notpenetrate through the substrate, and the resulting resonator deviceswill have a cross-section as shown in FIG. 3.

In all variations of the process 1100, the filter device is completed at1160. Actions that may occur at 1160 include depositing a passivationand tuning layer such as SiO₂ or Si₃O₄ over all or a portion of thedevice; forming bonding pads or solder bumps or other means for makingconnection between the device and external circuitry; excisingindividual devices from a wafer containing multiple devices; otherpackaging steps; and testing. Any dielectric layer deposited at 1160 orelsewhere in the process 1100 is deposited over all resonators. Anotheraction that may occur at 1160 is to tune the resonant frequencies of theresonators within the device by adding or removing metal or dielectricmaterial from the front side of the device. After the filter device iscompleted, the process ends at 1195.

Closing Comments

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andprocedures disclosed or claimed. Although many of the examples presentedherein involve specific combinations of method acts or system elements,it should be understood that those acts and those elements may becombined in other ways to accomplish the same objectives. With regard toflowcharts, additional and fewer steps may be taken, and the steps asshown may be combined or further refined to achieve the methodsdescribed herein. Acts, elements and features discussed only inconnection with one embodiment are not intended to be excluded from asimilar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set”of items may include one or more of such items. As used herein, whetherin the written description or the claims, the terms “comprising”,“including”, “carrying”, “having”, “containing”, “involving”, and thelike are to be understood to be open-ended, i.e., to mean including butnot limited to. Only the transitional phrases “consisting of” and“consisting essentially of”, respectively, are closed or semi-closedtransitional phrases with respect to claims. Use of ordinal terms suchas “first”, “second”, “third”, etc., in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements. As used herein, “and/or” means that the listed items arealternatives, but the alternatives also include any combination of thelisted items.

It is claimed:
 1. An acoustic filter device comprising: a substratehaving a surface; a lithium niobate piezoelectric plate having front andback surfaces and a thickness ts, the back surface attached to thesurface of the substrate except for portions of the lithium niobatepiezoelectric plate forming a plurality of diaphragms that spanrespective cavities in the substrate; a conductor pattern formed on thefront surface of the lithium niobate piezoelectric plate, the conductorpattern comprising a plurality of interdigital transducers (IDTs) of aplurality of acoustic resonators, interleaved fingers of each IDT of theplurality of IDTs disposed on a respective diaphragm of the plurality ofdiaphragms; and one or more dielectric layers deposited over all of theplurality of IDTs and the plurality of diaphragms, wherein a totaldeposited thickness of the one or more dielectric layers is the same forall of the plurality of acoustic resonators, wherein the interleavedfingers of all of the plurality of IDTs are aluminum with a commonthickness tm, where 0.12 ts<tm<0.32 ts, and each IDT of the plurality ofIDTs has a respective pitch p within one of following ranges: 2.8 ts to4.0 ts, 5.0 ts to 7.2 ts, and 8.2 ts to 10.6 ts.
 2. The device of claim1, wherein the one or more dielectric layers comprises apassivation/tuning layer.
 3. The device of claim 1, wherein the one ormore dielectric layers consists of a passivation/tuning layer.
 4. Thedevice of claim 1, wherein, for each IDT of the plurality of IDTs: tm/tsand p/ts define a point within one of the unshaded areas defined in FIG.7.
 5. The device of claim 1, wherein the lithium niobate piezoelectricplate and the plurality of IDTs are configured such that respectiveradio frequency signals applied to the plurality of IDTs exciterespective shear primary acoustic modes within the respectivediaphragms.
 6. The device of claim 1, wherein each diaphragm of theplurality of diaphragms is contiguous with the lithium niobatepiezoelectric plate around at least 50% of a perimeter of the respectivecavity.
 7. A method of fabricating an acoustic filter device,comprising: bonding a back surface of a lithium niobate piezoelectricplate to a surface of a substrate, portions of the lithium niobatepiezoelectric plate forming a plurality of diaphragms that spanrespective cavities in the substrate; forming a conductor pattern on afront surface of the lithium niobate piezoelectric plate, the conductorpattern comprising a plurality of interdigital transducers (IDTs) of aplurality of acoustic resonators, interleaved fingers of each IDT of theplurality of IDTs disposed on a respective diaphragm of the plurality ofdiaphragms; and depositing one or more dielectric layers over all of theplurality of IDTs and the plurality of diaphragms, wherein a totaldeposited thickness of the one or more dielectric layers is the same forall of the plurality of acoustic resonators, wherein the interleavedfingers of all of the plurality of IDTs are aluminum with a commonthickness tm, where 0.12 ts<tm<0.32 ts, and each IDT of the plurality ofIDTs has a respective pitch p within one of following ranges: 2.8 ts to4.0 ts, 5.0 ts to 7.2 ts, and 8.2 ts to 10.6 ts.
 8. The method of claim7, wherein depositing the one or more dielectric layers comprisesdepositing a passivation/tuning layer.
 9. The method of claim 7, whereindepositing the one or more dielectric layers consists of a depositing apassivation/tuning layer.
 10. The method of claim 7, wherein, for eachIDT of the plurality of IDTs: tm/ts and p/ts define a point within oneof the unshaded areas defined in FIG.
 7. 11. The method of claim 7,wherein the lithium niobate piezoelectric plate and the plurality ofIDTs are configured such that respective radio frequency signals appliedto the IDTs excite respective shear primary acoustic modes within therespective diaphragms.
 12. The method of claim 7, wherein each diaphragmof the plurality of diaphragms is contiguous with the piezoelectricplate around at least 50% of a perimeter of the respective cavity. 13.An acoustic resonator device comprising: a substrate having a surface; alithium niobate piezoelectric plate having front and back surfaces and athickness ts, the back surface attached to the surface of the substrateexcept for a portion of the lithium niobate piezoelectric plate forminga diaphragm that spans a cavity in the substrate; and an interdigitaltransducer (IDT) formed on the front surface of the lithium niobatepiezoelectric plate with interleaved fingers of the IDT disposed on thediaphragm, wherein the interleaved fingers are aluminum with a thicknesstm, where 0.12 ts≤tm≤0.32 ts, and a pitch p of the interleaved fingersfalls within one of the following ranges: 2.8 ts to 4.0 ts, 5.0 ts to7.2 ts, and 8.2 ts to 10.6 ts.
 14. The device of claim 13, wherein thesingle crystal piezoelectric plate and the IDT are configured such thata radio frequency signal applied to the IDT excites a shear primaryacoustic mode within the diaphragm.
 15. The device of claim 13, whereinthe diaphragm is contiguous with the piezoelectric plate around at least50% of a perimeter of the cavity.
 16. The device of claim 13, whereintm/ts and p/ts define a point within one of the unshaded areas definedin FIG.
 7. 17. The device of claim 13, further comprising a dielectricpassivation/tuning layer deposited over the IDT and the front side ofthe piezoelectric plate.